The safety critical elements are those parts of an installation or plants that can be subjected to failure and substantially cause, contribute, prevent or help recover from a major accident hazard or event. It is essential to develop performance standards for the identified SCEs. The SCE life cycle management involves alignment of maintenance, inspection and testing and performance history to maintain SCE in suitable working conditions. Continual monitoring helps in conformance of SCEs by Performance Standards.
The following steps as depicted in are used for the identification of SCEs:
- Identify the systems and elements of the facility
- Review the list of Major Accident Events (MAE) , developed in the HAZID Study and Major Accident Events List, and identify the systems associated with each MAE;
- Assess the criticality of each system by determining whether the failure of the system could cause a MAE or contribute substantially to a MAE. Any system found to be safety critical on this basis is deemed to be safety critical for reasons of integrity, i.e. if their integrity is maintained, then the MAE cannot occur;
- For any system found not to be safety critical above, assess whether the system prevents or limits the effects of a MAE. Any system or equipment item found to be safety critical on this basis is deemed to be safety critical for reasons of control and mitigation as emergency response to a MAE; and
- Once identified, the SCEs are categorised as follows:
- Integrity; and/or
- Control and mitigation.
This process allows the identification of all SCEs associated with the facility as well as detailing those systems deemed not safety critical with regard to causing or mitigating a MAE.
Note: There are several systems whose failed integrity could lead to serious harm/injury,
e.g. pressurised release, exposure to chemicals, etc, but not necessarily a MAE. These systems are not identified as an SCE, as the consequence due to failure of their integrity is not severe enough to lead or contribute ‘substantially’ to a MAE. This determination has been made by reference to the results of the HAZID, and engineering judgement and experience.
The recommended model for developing Safety Critical Element Performance Standards is based on the Functionality, Availability, Reliability, Survivability and Interaction (FARSI) format where:
Functionality refers to the purpose that the Safety Critical Element has to be able to perform to prevent, detect or mitigate a hazardous event or to protect people.
Availability refers to the proportion of the time that the Safety Critical Element will be required to perform on demand.
Reliability refers to how likely the Safety Critical Element is to perform on demand.
Survivability refers to how the Safety Critical Element will perform after a major accident has occurred, i.e. how well it will survive a fire, explosion, dropped object, etc.
Interaction refers to the way that the Safety Critical Element in question is dependent upon other SCEs to operate or otherwise interacts with other SCEs